Most image display units having picture elements arranged so as to form a matrix, which are represented by liquid crystal display units provided with variety of circuits having prescribed functions such as a driver circuit for driving the display units and a controller circuit for controlling the driver circuit. The arrangement and size of these circuits vary with the configuration of the image display units, but these circuits are indispensable to visualize information according to all kinds of media such as TV. Here, the description will be given of the arrangement of a liquid crystal display unit, particularly an active matrix-type liquid crystal display unit.
The above-mentioned liquid crystal display unit is formed with a scanning signal line crossing a data signal line on the surface of a substrate which is a member of a liquid crystal display (LCD), and picture elements with a composition where the liquid crystal is mounted between a picture element electrode and a counter electrode is formed in the vicinity of an intersection of the respective signal lines. These picture elements which are arranged so as to form a matrix constitutes a picture element section. Each picture element is driven by a picture element driving element such as a thin film transistor (hereinafter, referred as TFT) which is formed in the vicinity of the intersection of the scanning signal line and the data signal line.
Two kinds of drivers are used as the driver circuit of the liquid crystal display unit: (1) a data driver also referred as a source driver, which receives a video signal and which samples the signal so as to output sampled image data obtained during one horizontal scanning period, in other words, in one horizontal line period; and (2) a scanning driver also referred as a gate driver, which specifies a storing picture element of the image data transmitted to the picture element section. Although each arrangement in the drivers varies with specifications of liquid crystal displays, the data driver is consisted of, for example, a shift register, a sampling circuit, a transfer circuit, an output buffer, etc., and the scanning driver is consisted of, for example, a shift register, a level shifter, an output buffer, etc.
As an example, the arrangement and operation of the data driver will be described referring to FIGS. 66 through 69. Here, FIG. 66 is a block diagram of a typical data driver adopted a line successive scanning system which is used for a liquid crystal display unit adopted an active matrix, FIG. 67 is an example of a timing chart of each section in FIG. 66, FIG. 68 is a block diagram of the data driver adopted a point successive scanning system, and FIG. 69 is an example of a timing chart of each section in FIG. 68.
As shown in FIG. 66, in the data driver adopted the line successive scanning system, a clock signal (hereinafter, referred as CLP) and a start signal (hereinafter, referred as STP) are inputted into a sampling signal generating circuit 101 in the data driver. For example, if a number of the data driver outputs is N, the sampling signal generating circuit 101 includes the shift register circuit with N steps. The STP, which starts sampling data of one horizontal scanning period, is inputted to the sampling signal generating circuit 101 so that sampling pulses C.sub.1 to C.sub.N are outputted from each output section of the circuit 101 in accordance with a timing of the CLP. A video signal is sampled in a sampling circuit 102 by the sampling pulses C.sub.1 to C.sub.N outputted from the sampling signal generating circuit 101, and the sampled signal data Z.sub.1 to Z.sub.N are successively written to a sampling capacitor. The signal data of one horizontal scanning period written to the sampling capacitor are outputted from a transfer circuit 103 through an output buffer 104 to a data signal line based upon a transfer signal (hereinafter, referred as TRF). The data of one horizontal scanning period are stored into a predetermined picture element of the liquid crystal display panel by applying a scanning pulse from a scanning driver to a scanning signal line in accordance with a data transfer timing to the data signal line.
Furthermore, while the signal data are being transferred to the liquid crystal display, the video signal of next one horizontal scanning period are being sampled. Then, before the TRF signal, which transfers the newly sampled data to an output buffer 104, is inputted into the transfer circuit 103, a discharge signal (hereinafter referred as DIS) is applied to the output buffer 104, and the former signal data are deleted from the data signal line.
Meanwhile, in a data driver which adopts a point successive scanning system, as shown in FIG. 68, the video signal is sampled in accordance with the sampling pulses C.sub.1 to C.sub.N outputted from the sampling signal generating circuit 101 in like manner of the data driver which adopts the line successive scanning system. However, the sampled signal is immediately transferred to the data signal line without being written to the sampling capacitor. The data of one horizontal scanning period are stored in predetermined picture elements of the liquid crystal display by applying the scanning pulse from the scanning driver to the scanning signal line in accordance with a timing of the data transfer to the data signal line in similar manner as the above.
Here, in the point successive scanning system, since as to a picture element where the signal data whose sampling timing is last in one horizontal scanning period are stored, the time from a point where the signal data are outputted to the data signal line to a period the scanning pulse goes OFF is short, when carrier mobility of a picture element driving element as an active device is low, a charging time to an picture element is insufficient and the signal data cannot be written sufficiently. Therefore, the point successive scanning system inevitably requires an active device whose carrier mobility is high.
In general, in the line successive scanning system, an amorphous silicon thin film transistor (hereinafter, referred as a-SiTFT) is used as the active device in the liquid crystal display unit, and in the point successive scanning system, a polycrystal silicon thin film transistor (hereinafter, referred as p-SiTFT) whose carrier mobility is higher than that of a-SiTFT is used.
The driver circuit (the data driver or the scanning driver) is connected to a substrate where the picture element section has been formed by generally using a TAB (tape automated bonding) method. In this method, the connection is made by gang-bonding a driver LSI (large scale integration) as the driver circuit to a flexible tape and by, for example, thermocompression bonding the flexible tape to a glass substrate which is included in the liquid crystal display panel.
However, in recent years, as a liquid crystal display unit are highly refined, its picture element pitch becomes narrower. As a result, a picture element pitch, which is less than a connection limit pitch of the above TAB, is required, and a so-called COG (chip on glass) method where a driver LSI is directly mounted on the glass substrate of the liquid crystal display is used.
In addition, in the case where the p-SiTFT is used as the active device, its carrier mobility .mu. is obtained as .mu..gtoreq.5 cm.sup.2 /V.multidot.sec. As a result, since the carrier mobility .mu. is 10 to 1000 times higher than that of a-SiTFT, the picture element section and the above-mentioned driver circuit can be monolithically formed on the glass substrate of the display panel.
As mentioned above, when the driver circuit is directly mounted on the glass substrate by the COG or is monolithically formed on the glass substrate, in the case where the driver circuit is mounted on the glass substrate, it is inevitable to wire not only the scanning signal line and the data signal line but also a power source line for supplying electric power from external power supply to the driver circuit and a plurality of signal lines 152 . . . (see FIG. 70) for inputting a clock signal, a start signal, a video signal, etc. to the driver circuit.
Here, the reference symbols S (S.sub.1, S.sub.2 . . . ) of FIG. 70 represent a shift register circuit of the sampling signal generating circuit 101, and the reference numerals 102a . . . of FIG. 70 represent the sampling switches of the sampling circuit 102.
If wiring material Ta or TaNx, which is used for conventional a-SiTFT-LCD, is used as wiring material of the power source line and the signal line 152, an image quality differs on the right end and on the left end of the screen, thereby arising a problem such as the lowering of the display characteristic, etc.
This problem is synergistically arisen because of the following two reasons.
Namely, one is a material characteristic that in the case where a resistivity .rho. of the Ta or the TaNx is 25 to 30 .mu..OMEGA..multidot.cm, and the same wiring material is used for wiring, if the wiring width is 100 .mu.m and the film thickness is 300 nm, the wiring resistance becomes 100.OMEGA. per centimeter.
As shown in FIG. 71, the other is that each signal line 152 such as a start signal line 152 for inputting the start signal to the shift register circuit S(S.sub.1, S.sub.2 . . . ) and a video signal line 152 for inputting the video signal to the sampling switch 102a . . . is connected, and one connecting pad 153, which is used for the connection with an external circuit substrate, is provided for each input signal. Here, the reference numeral 155 of FIG. 71 represents the substrate.
The concrete description will illustrate a video signal as the signal line 152. In a liquid crystal display unit whose diagonal is 25 cm or so, when the video signal line is connected from end to end in a crosswise direction, its wiring length becomes approximately 20 cm and wiring resistance of the signal line becomes 2 k.OMEGA.. Further, even in a liquid crystal display unit whose diagonal is 13 cm or so, the wiring resistance of the signal line becomes 1 k.OMEGA.. When a video signal is transmitted through such a signal line having high resistance, impedance increases. As shown in FIG. 72, as a signal, which has a band A in a connecting pad 153 as a signal input terminal, is transmitted through the signal line 152, its band characteristic becomes worse A.fwdarw.B.fwdarw.C, and at the end of the signal line 152, the signal shows a band characteristic like D. Then such a phenomenon causes the trouble that an image quality greatly differs on the right end and on the left end of the screen, thereby arising a problem that images having uniform quality cannot be displayed.
In addition, if this phenomenon occurs in the start signal line 152 or the clock signal line 152 for inputting the start signal to each shift register circuit S (S.sub.1, S.sub.2 . . . ) of the sampling signal generating circuit 101, or a line for transmitting a shift pulse to a next step in the shift register circuit, as shown in FIGS. 73(a) through 73(c), a waveform of the sampling signal outputted from the sampling circuit 102 is changed from its early state such that its rising part and falling part become duller as the signal is transmitted through the steps of the sampling signal generating circuit 101. In other words, a waveform deterioration occurs. Namely, the sampling signal outputted from the shift register circuit S.sub.1 on the first step of the sampling signal generating circuit 101 is represented by a waveform a in FIG. 73(a), but as shown in FIG. 73(b), the sampling signal outputted from the shift register circuit Sn on the "n"th step is represented by a waveform b' which is obtained because of the deterioration of the primary waveform b. Furthermore, as shown in FIG. 73(c), the sampling signal outputted from the shift register circuit Sm on the "m"th step is represented by a waveform c' which is obtained because of the deterioration of the primary waveform c. Therefore, there arises a situation that the sampling phase is displaced from its regular position or the sampling signal is not generated, thereby arising a problem that images of good quality cannot be displayed.
Here, Japanese Laid-Open Patent Publication No. 398385/191992 (Tokukaihei 4-348385) has disclosed an art which compensates for a waveform deterioration of a signal data by detecting an electric current flowing to a display panel and by controlling an applied voltage to the display panel according to an amount of the electric current. However, the art disclosed in the above publication is especially applicable to an image display unit adopted a simple matrix driving, but is not applicable to a waveform deterioration caused by a monolithic or COG (chip on glass) driver in, for example, an image display unit having an active device adopted an active matrix driving system, namely an irregular operation in the circuit of the device.
In addition, the above-mentioned increase in impedance (wiring resistance) may occurs in not only the signal line 152 but also the power source line. As shown in FIG. 74, as the distance from the connecting pad increases, the source voltage lowers. For example, even if a source current is 1 mA, when the current flows through the power source line of 2 k.OMEGA., the source voltage is lowered 2 V. Such a fall of the source voltage causes irregular operations including non-operation of each circuit connected to the power source line, fluctuation in the signal level, etc., thereby causing a fall of the display characteristic.
As the size of the screen in the liquid crystal display panel becomes larger, the above-mentioned problems surely become more conspicuous.
As shown in FIG. 75, in order to prevent the wiring impedance from increasing, it has been conventionally considered to enlarge the wiring width of the signal line 152'. In this case, the wiring resistance can be limited to about 1/10 by, for example, changing the wiring width of 100 .mu.m to ten times, namely, 1 mm. However, in this manner, for example, in the case where the impedance of the signal line 152' is further lowered, the signal line 152' needs to be thickened more. As a result, as shown in FIG. 76, when the connecting pads 153 . . . are arranged on the periphery of the substrate 155 so as to make a connection with an external circuit substrate, an area of a non-picture element section relative to the picture element on the display unit becomes larger with an increase in areas of the wiring and the connecting pads. In addition, there arises disadvantages such as an increase in overlapping capacity of wires depending upon a case, an increase in a signal leak between wirings.
In addition, in order to prevent the wiring impedance from increasing, it is considered that aluminum (Al) or an aluminum alloy (Al-Si) is used as wiring material. For example, if Al-Si whose resistivity .rho. equals 5 .mu..OMEGA..multidot.cm (.rho.=5 .mu..OMEGA..multidot.cm) is used as the wiring material so as to be wired with the aforementioned wiring width and film thickness, the wiring resistance is limited to about 1/6 of that obtained in the case where Ta or TaNx is used. For example, when the above Al-Si is applied to an image display unit whose diagonal is 25 cm or so, the total wiring resistance becomes approximately 330 .OMEGA., and when applied to an image display unit whose diagonal is 13 cm or so, becomes 170 .OMEGA.. In this case, if the same load condition (source current=1 mA) as the above is taken into consideration, in the case where the wiring material is used in an image display unit whose diagonal is 25 cm, a voltage drop due to the wiring material can be limited to about 330 mV.
However, it is a rare case that the source current is constantly kept 1 mA, so the source current fluctuate at a high frequency by an ON/OFF operation of each active device such as a transistor which is connected to the power source line, and as shown in FIG. 77, a voltage waveform which fluctuates at a high frequency is shown in a certain point of the power source line. In this case, if it is considered that the ratio of the fluctuation in the signal data (Z.sub.1 to Z.sub.N of FIG. 66 or FIG. 68) to the fluctuation in voltage is 1:1, in a certain picture element on the screen, for example, the brightness at the time of the maximum fluctuation in voltage differs greatly from that at the time of the minimum fluctuation in voltage. The more a number of active elements to be turned ON/OFF is or the higher the impedance of the power source line is, the more outstanding this phenomenon is.
Here, the description discusses the influence of the fluctuation in source voltage on the signal data. However, the fluctuation in source voltage, that is, a high-frequency noise generated in the power source line has influence on not only the signal data but also the other signals such as the clock signal. Furthermore, the high-frequency noise surely causes not only the fluctuation in a voltage level of a signal but also the other elements such as a response time (operating speed) and a faulty operation of a circuit.
In order to limit the high-frequency noise, in the conventional art, as shown in FIG. 78, a capacitor 164 is mounted on the outside of a display substrate 155 where picture elements and a power source line 161 are formed, for example, on a flexible substrate 163 so that the capacitor 164 is connected to the power source line 161. Although this method is applicable to the purpose in reducing high-frequency noises generated on the outside of the display substrate 155, it is not inadequate to reduce high-frequency noises generating in the display substrate 155 including the fluctuation in voltage caused in accordance with the fluctuation in electric current caused by turning on/off each active device connected to the power source line 161.
The above description illustrated the image display unit represented by the liquid crystal display unit. The above-mentioned problems, such as the fluctuation in a signal level which causes an irregular operation of a circuit, a signal delay due to a signal waveform deterioration, an increase in high-frequency noises, are related to semiconductor devices where circuits including a semiconductor active device and long electrically conductive lines, such as a power source line for supplying a source power to the circuits, a signal line for inputting a signal to the circuits, are mounted on one substrate.